Note For complete syntax and usage information for the switch commands used in this chapter, first look at the Cisco Catalyst 4500 Series Switch Command Reference and related publications at this location:

If the command is not found in the Catalyst 4500 Series Switch Command Reference, it will be found in the larger Cisco IOS library. Refer to the Cisco IOS Command Reference and related publications at this location:

IP Unnumbered Interface Support with DHCP Server and Relay Agent

The IP unnumbered interface configuration allows you to enable IP processing on an interface without assigning it an explicit IP address. The IP unnumbered interface can “borrow” the IP address from another interface that is already configured on the Catalyst 4500 series switch, which conserves network and address space. When used with the DHCP server/relay agent, this feature allows a host address assigned by the DHCP server to be learned dynamically at the DHCP relay agent.

Figure 14-1 shows a sample network topology implementing the IP Unnumbered Interface feature. In this topology, IP routes are dynamically established by the aggregation switch when the DHCP server assigns IP addresses to the hosts.

DHCP Option 82

DHCP provides a framework for passing configuration information to hosts on a TCP/IP network. Configuration parameters and other control information are carried in tagged data items that are stored in the options field of the DHCP message. The data items are also called options. Option 82 is organized as a single DHCP option that contains information known by the relay agent.

The IP Unnumbered Interface feature communicates information to the DHCP server using a suboption of the DHCP relay agent information option called agent remote ID. The information sent in the agent remote ID includes an IP address identifying the relay agent and information about the interface and the connection over which the DHCP request entered. The DHCP server can use this information to make IP address assignments and security policy decisions.

Figure 14-2 shows the agent remote ID suboption format that is used with the IP Unnumbered Interfaces feature.

To configure IP unnumbered interface support with connected host polling, perform this task:

Command

Purpose

Step 1

Switch#
enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

Switch#
configure terminal

Enters global configuration mode.

Step 3

Switch(config)#
interface vlan vlan-id

Enters interface configuration mode and the interface to be configured as a tunnel port.

Step 4

Switch(config-if)# ip unnumbered type number poll

Enables IP processing and connected host polling on an interface without assigning an explicit IP address to the interface

type and number specify another interface on which the switch has an assigned IP address. The interface specified cannot be another unnumbered interface.

The type argument can have the values: loopback, fastethernet, gigabitethernet, svi, and portchannel.

Step 5

Switch(config-if)#
exit

Returns to global configuration mode.

Step 6

Switch(config)# ip arp poll queue <10-10000>

Configures the global backlog queue of host addresses to be discovered.

Default for the queue size is 1000.

Step 7

Switch(config)# ip arp poll rate <10-10000>

Configures the maximum number of ARP requests sent over unnumbered interfaces.

Default number of ARP requests is 1000 packet per second.

Step 8

Switch(config)# end

Returns to privileged EXEC mode.

Step 9

Switch#
show running-config

Verifies that IP unnumbered support has been configured correctly.

The following example shows how to enable IP processing and connected host polling on Fast Ethernet interface 6/2. It also shows how to set the global backlog queue to 2000 and the maximum number of ARP requests to 500:

Troubleshooting IP Unnumbered Interface

To understand how to debug connect host polling, see the Cisco IOS documentation of the debug arp command on cisco.com.

When an IP unnumbered interface shares the IP address of a loopback interface whose prefix is advertised in an OSPF network, you must modify the loopback interface as a point-to-point interface. Otherwise, only the loopback interface host route is advertised to an OSPF neighbor.